It is standard, for example from old East German patent 295,052 of K. Tschirschky, German patent 2,748,737 of H. Grass, and German utility models 9,108,281 and 9,304,064, to provide a filter over a vent opening of a piece of power-ventilated electrical equipment. The filter prevents the equipment's fan from sucking in particles and also prevents particles from getting into the equipment through the intake and/or outlet hole when the fan is not running.
Such equipment is frequently provided outside or in locations where it might even be sprayed with water. Thus it is important that the filter assembly not provide a path for water to enter the equipment. This is typically done as seen in the above-cited patent references by providing angled louver openings. While such construction does largely eliminate the possibility of water getting into the device and wetting the filter medium, it substantially reduces the flow cross section of the opening, thereby reducing the capacity of the device.